Seawater contains approximately 2-100
ppt of silver, and
the surface concentration may be even lower. River water generally
contains approximately 0.3-1 ppb of silver. The phytoplankton
concentration is 0.1-1 ppm (dry mass), leading to a 104-105
bio concentration factor in seawater. In oyster tissue concentrations of
approximately 890 ppm (dry mass) were found.
Dissolved in water silver mainly occurs as Ag+ (aq), and in
seawater as AgCl2- (aq).
In what way and in what form does silver react with water?
Silver does not react with pure water. Is is
stable in both water and air. Moreover, it is acid and base resistant,
but it corrodes when it comes in contact with
sulphur compounds.
Solubility of silver and silver compounds
Under normal conditions silver is water insoluble.
This also applies to a number of silver compounds, such as silver
sulphide. Some other sulphur compounds are more or less water soluble.
For example, silver chloride has a water solubility of 0.1 mg/L,
maximum. Silver nitrate has a water solubility of 2450 g/L. Silver
fluorides are generally water soluble, but other silver halogens are
not.
Why is silver present in water?
Silver mainly occurs in argenite and stephanite,
from which it is released through weathering. In soils it is mainly
present in sulphide minerals. Naturally occurring pure silver is
extremely rare and is probably formes through the following reaction
mechanism:
3 Ag2S + 2 H2O -> 6 Ag + 2 H2S
+ SO2
Besides gold, silver is the most bendable of all
metals. It is known for its high thermal and electrical conductivity,
its reflective power and its white colour. It is applied for example in
copper,
nickel and
tungsten alloys.
Amalgam is a silver alloy with a high
mercury content. In
electronics, silver is applied for outlets. Commonly known are
applications in jewellery, coins and cutlery. Objects are often provided
with a silver layer, including mirrors.
Silver compounds play an important role in photo and film production,
and are applied in developing chemicals. It serves as a catalyser in
many chemical processes. Silver oxides are applied in battery
production.
Colouring agents for food stuffs, preservatives and
disinfectants may contain silver. Silver is added to the atmosphere
as AgI to prevent hail. It is generally a by-product of metal refinery,
and may be recycled.
The 110mAg isotope is applied in nuclear physics.
What are the environmental effects of silver
in water?
Silver is not a dietary requirement for organisms.
It may even be lethal to bacteria, and it inhibits fungi reproduction.
This is mainly caused by Ag+ ions. At oral silver uptake by
warm-blooded organisms, about 10% is absorbed. Mammal flesh contains
approximately 4-24 ppb (dry mass) of silver. Mammals take up silver
mainly through plant feed.
Plants may absorb silver, although it has no biological use. Values of
between 0.03 and 0.5 ppm (dry mass) were measured in the past. Fungi and
green algae may even obtain a silver content of 200 ppm (dry mass).
Soils do not contain great amounts of silver. However, areas rich in
minerals may contain higher amounts. In mining areas soil silver amounts
of up to 44 ppm were found. In normal air-dried soil concentrations do
not exceed 100 ppb.
In water silver and silver compounds are toxic to micro organisms. Fish
contain approximately 11 ppm of silver. Silver toxicity to fish is
reduced by water. Depending on water hardness, the lethal concentration
for freshwater fish lies between 4 and 280 ppm. Freshwater plants
tolerate between 30 and 7,500 ppb silver, depending on the species. The
lethal concentration for daphnia is approximately 0.25 ppb, and for
amphipods at 4,500 ppb.
Naturally occurring silver concentrations in soil and surface water do
not normally cause any
environmental problems.
LD50 values
were determined for various silver compounds. For silver oxide the LD50
for rats at oral intake is 2820 mg/kg, and for silver nitrate the LD50
for mice at oral intake is 50 mg/kg. For dogs 2.3 g of silver nitrate is
lethal. Silver difluoride is extremely toxic, and it is also excellently
water soluble. Silver toxicity has a very broad spectrum.
Silver is not known to be carcinogenic. However, when it is directly
implanted under the skin of animals it can cause cancer.
Silver has two stable and twenty four instable (radioactive) isotopes.
What are the health effects of silver in water?
Silver is not a dietary requirement for humans. The body of an adult
contains approximately 2 mg of silver. Our daily intake of silver is
20-80 μg, of which approximately 10% is absorbed. These amounts are
not health threatening. In larger amounts, some silver compounds may be
toxic, because silver ions have a high affinity for sulphur hydryl and
amino groups, and therefore complexation with amino acids, nucleic acids
and other compounds occurs in the body. We known the mechanism of
toxicity, so we also known a number of detoxification methods. The toxic
mechanism is relatively small at oral uptake, because of the low
absorption capacity of the body for silver.
Silver that ends up in the body is generally deposited in connective
tissue, skin and eyes and causes a gray to black colouring. Within 50
years, one is able to accumulate approximately 9 mg of silver.
The drinking water guideline for silver is 0.05 mg/L, if a guideline is
fixed at all. This is mainly because silver may bind to sulphur in food
in boiling water. Silver oxide is harmful upon swallowing, because it
irritates the eyes, respiratory tract and skin. Silver nitrate is much
more harmful, because it is a strong oxidant. It causes corrosion and at
oral uptake it leads to vomiting, dizziness and diarrhoea. At silver
salt uptake the body may protect itself by converting them to insoluble
silver chlorides.
Silver is a bactericide, and may therefore be applied in
water
disinfection.
Which water purification technologies can be applied to remove
silver from water?
Ionic silver may be removed from water by
ion exchange. Some silver compounds
may precipitate by coagulation.
Two other efficient methods include
active carbon filtration
and sand filtration.
Silver is applied in water
purification for
swimming
pool water disinfection. Only small amounts are applied that are not
a health hazard.
Literature and the
other elements and their interaction with water
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